There is a continuing need for high performance thermoset polymers. The properties that are usually important are high service use temperatures, better physical properties and low moisture uptake.
Epoxy resins fall short of performance requirements for advanced composites because of their insufficient thermal stability and high moisture uptake. This is probably due to the large number of hydroxy groups in these polymers.
One alternative to epoxides is a class of polymers that is referred to as polyimides. However, these polymers are often difficult to use due to their limited solubility and their limited thermal processability. Most fully cyclized polyimides are either insoluble or soluble only in high boiling solvents such as N-methylpyrrolidinone and the like. Most polyimides decompose before they melt and thus, can not be melt processed.
Another alternative is a class of polymers known as addition polyimides. Polymers have been developed based on bismaleimides and bisnadimides as shown, for example, in U.S. Pat. No. 4,239,883 to Stenzenberger. These polymers are made by first preparing the bismaleimide or bisnadimide prepolymer and then heating the prepolymer to a temperature between about 100.degree. to 400.degree. C. These prepolymers are thought to react by free radical addition polymerization of the unsaturated bonds on the bismaleimide or bisnadimide at the elevated temperatures.
There are several different chemical linkages that can be used to join the reactive maleimide or nadimide ends. One of the most important connections has been through arylamide chains because of the excellent thermal stability of these chains. Addition polyimides having these connecting chains are commercially available.
While these addition polyimides have many desirable properties, they have been difficult to make. The conventional synthesis of these materials involves the reaction of a diamine with maleic anhydride or nadic anhydride followed by cyclization of the resultant adduct with heat or chemical reagents. This process is illustrated with nadic anhydride as follows: ##STR1##
For the preparation of arylamide derived materials (where R contains an aromatic amide group), the use of moisture sensitive acid chlorides have been needed to obtain the amide linkages. In many cases the reduction of aromatic nitro groups to aromatic amines have been employed as a reaction step. These reduction reactions are generally not complete and require several recrystallizations in order to obtain bifunctional bismaleimides or bisnadimides of sufficient purity for effective network formation.
In all of these processes, there is a final cyclization step. Without the cyclization step, the end nadimide group is not sufficiently active. Nonreactive end groups could lead to less than optimal performance in the final cured composite structure. Thus, there is a continuing need for improved processes for making addition polyimide polymers. It would be desirable if the process could avoid the use of acid chloride reactions, nitro reductions, and incomplete cyclization.